LOOSE LIPS CAN SINK YOUR ENERGY SHIP . . . .
Sealing Energy Savings
By Glenn E. Gabryel, CR Chicago Rawhide
     The key role of rubber radial lip seals to retain lubricants and exclude contamination in mechanical systems is well understood by most engineers and mechanical technicians. However, one often overlooked seal operating parameter is the amount of power loss caused by the seal. 
     Similar to bearings, radial shaft seals depend on an oil film to support the lip load. These seals perform by controlling this oil film thickness under the lip, and must do so over a wide range of application conditions. However, in machine systems, shaft seals function in a boundary lubrication mode, where the wear track of the seal lip is in intermittent contact with high peaks of the shaft surface texture. 

 

     This process is important in the early hours of seal operation to establish a complimentary directional pattern in the rubber material. The formation of this pattern (known as microasperties) is vital in creating a positive "pumping action" by the seal lip while simultaneously smoothing the shaft. However, in conjunction with viscous shear of the lubricant, it also means that seals are friction devices and do consume power. In any given system, the total amount of power consumed by contacting radial lip seals can be substantial. 
Power Consumption Variables
     Besides friction, the interference between lip and shaft, the elasticity of the rubber and the spring force (if used) creates a radial load which results in additional torque. Seal power consumption (rate of doing work) is calculated using the measured seal torque and shaft speed. Many factors can influence torque and hence power consumption.
     For example, at constant speed, seal torque and power consumption generally decrease as sump temperature increases, attributable to a reduction in fluid viscosity as it is heated. Figure 1 (below) demonstrates the corresponding decrease in power consumption as sump temperature increases for various shaft speeds. Note that raising the shaft speed for a given sump temperature results in more power consumption.
     Conversely, Figure 2 (below) illustrates the effect of increasing oil viscosity and shaft speed on power consumption. Consultation with a lubricant supplier to select the least viscosity for the job can help to minimize system energy consumption. Lubricant level also affects power loss.
     Figure 3 (below) shows that lowering the sump fill level reduces power consumption. However, few things are free because this generally decreases heat transfer in the seal lip contact zone, typically .025"/.64 mm wide after a break-in period. Typically, the temperature rises in this critical area as a sump fill level falls. The resulting thermal stress lowers seal life. Therefore, a compromise must be reached according to the designer or users priorities.
Seal Design Effects
     Torque and power consumption values do vary with the seal design and material. If your company's using first generation shaft seals with leather or rubber lips, one fast way to reduce torque loss is to change to a current design with a lighter lip (also check the shaft surface quality). The "old" assembled designs functioned using brute force and tended to have aggressive springs, wide contact bands, large interferences and high radial loads. Modern "new" molded seals work longer and more effectively with narrow contact bands and minimal radial loads that are uniform over time. 
     However, there are still differences even between "new" designs. The decisions that seal engineers routinely make regarding design parameters such as lip radial cross-section (and therefore radial load) can result in considerable variation in torque drag. As a result, seals from two different manufacturers made for the same shaft and bore dimensions can have quite different power consumption characteristics.
     Furthermore, the state of the art in rubber lip seal manufacturing calls for "flashless" molding. By utilizing precision tooling, this means the seal lips are finished in the molding operation. Quality is therefore manufactured into the seal rather than inspected in afterwards. Previous methods rely on knife trimming to achieve the proper lip geometry. This secondary operation can introduce process variability which affects quality and power usage.
Seal Design Options
  •      Sometimes the right answer to minimizing radial shaft seal related power loss is the decision not to use one. If the primary sealing objective is to exclude contamination in systems with high viscosity lubricants (i.e. greases) then a device known as a V-ring may be a better solution. This all-rubber seal design grips the shaft radially while a lightly loaded angled lip seals against an axial counterface, offering several advantages:
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    • Very low torque drag and power loss.
    • Minimal heat build-up.
    • Does not require fine countersurface finish.
    • Highly effective centrifugal slinger action.
    • Extended service life potential.
         The large body section rotates with the shaft, providing centrifugal force at the angled seal lip. Compared to radial lip seals, V-rings have very low torque drag and power loss. Fluid sprays, dusts and particles are then flung away from the lip contact point. Since the V-ring lip works primarily by this slinger effect, it requires only light axial loading. Field experience suggests that the combination of a radial seal to retain oil and a V-ring to exclude contamination lasts longer and is considerably more effective than a single shaft seal with dust-lip. At lower surface speeds, the V-ring lip remains in positive contact with the countersurface.
         At approximately 3,000 FPM, the lip lifts off the countersurface and torque drag decreases to nearly zero. However, at this point, centrifugal forces create a very effective contamination barrier. As speed increases, supplemental mechanical radial and axial retention are needed to maintain the V-ring's position. V-rings are not suitable for every application. For example, they cannot retain light viscosity lubricants. Pressure and submerged conditions are also limiting factors.
    Avoid Adding Energy Consuming Load
         Some designers believe that if one seal is good, then multiple elements must be better. On occasion this may be needed to separate two media. Tandem lip elements may also be useful to maximize contaminant exclusion. However, every extra contacting radial lip adds both torque drag and heat load. The outer lips generally receive less lubrication (even if grease packed) and as a result they can harden and prematurely wear to the nominal shaft diameter. Paradoxically, this can actually trap contaminants between seal elements and further accelerate wear.
         It is also important to use the right seal for the job. Some types, especially those that seal primarily by interference, are better suited to static operation or a limited range of intermittent motion. Trying to save money by using an o-ring to seal a gearbox output shaft turning at 1,500 FPM will result in high power consumption and short seal life. Some applications (i.e. pumps) may require the use of compression braided packings, but their power loss is typically not a major concern.
    Change Seal Materials?
         The seal compound does have an effect on power loss, but contrary to expectation, changing seal materials may not significantly improve system energy consumption, and instead can work against you. For example, while PTFE (Teflon) has a low coefficient of friction, PTFE shaft seals usually have high radial loads and wide seal lip contact areas resulting in considerable torque drag. PTFE is better used for objectives other than energy savings.
         Special rubber compounds are of particular value when the application conditions are known and constant and production volumes are relatively high. Due to economics and variable end user conditions, seal specifiers do not often have this luxury. Consultation with the seal supplier or your local PT/MC distributor is important since modifying a compound to maximize one parameter usually means reduced capability in others. As an example, incorrectly utilizing special fillers to enhance low friction properties in rubber radial shaft seals can improve dry wear resistance, but decrease tear strength and sealing performance.
    This article is furnished courtesy of PTDA

     

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